There are many different things you might need in an emergency. Here are a few categories you'll want to look for.
Emergency Food & Water Supplies - Bottled water is definitely a good idea. It's good to have that available even in a normal situation, not just an emergency. Other things to consider are a 3-day supply of food per person and granola or energy bars.
Emergency Shelter Supplies - This includes things like ponchos, a blanket, a solar blanket, body warmer pads and even umbrellas.
Emergency Lighting - If you're stuck in the dark, you'll want things like road flares, lantern lights, light sticks, flashlights and lots of extra batteries for each of these items you include.
Emergency Radios - Having a solar or hand-cranked am/fm radio can sometimes come in handy, especially if it has a way to listen to emergency broadcasts. Some lanterns come with radios built in which is good, but once again make sure you have extra batteries if that's what is uses.
Emergency First Aid Supplies - Like bottled water, it never hurts to have a first aid kit on hand. Buy one in a store or build your own with compresses, antiseptics, bandages, gauze pads, rolls, basic pain killer and other common first aid products. If you or someone you frequently travel have asthma, diabetes or some other situation that might require medicines, it's a good idea to talk to your doctor about ways to include them in your roadside kit as well.